An amplifier stage is known to have variable gain by varying the gain of one or more individual intermediate stages within the amplifier stage. An amplifier stage includes two or more individual intermediate gain stages. According to one method, by varying the gain of one or more of the individual intermediate stages of the amplifier stage, the overall gain of the amplifier stage is varied. For example, each of the individual intermediate gain stages typically include a transistor based amplifier for providing gain. The gain of an individual intermediate stage is varied by changing the gain of the intermediate stage transistor; typically, by varying the DC bias of the intermediate stage transistor. This may be accomplished, for example, by changing the DC bias current through the base of a bipolar transistor, or by varying the DC bias voltage in the gate of a junction field-effect transistor.
However, changing the DC bias point of any one of the intermediate stage transistors to vary the gain of one or more intermediate amplifier stages within the variable gain amplifier may affect the input and output impedances of the amplifier stage, and may also affect the noise figure and the third order harmonic intercept of the amplifier stage. The noise figure of an amplifier stage is the ratio between the signal to noise ratio of the input of the amplifier stage and the signal to noise ratio of the output of the amplifier stage. The noise figure of an amplifier stage is an indication of the amount of noise added to the input signal by the amplifier stage. The third order intercept harmonic is related to the sum and difference of two or more signals to be amplified. Changes in the input and output impedances may cause an impedance mismatch with an input source and output load. Further, changes in the noise figure and in the third order intercept harmonic may result in the generation of excessive noise for the variable gain control amplifier.
A second method for varying the gain of an amplifier stage is to switch between different intermediate gain stages of an amplifier stage such as a cascade or cascode amplifier stage. A cascade amplifier stage usually consists of a number of intermediate stages where the output of a previous stage is coupled to the input of a subsequent stage. In addition to providing gain, the first, or input stage is usually required to provide a high input resistance in order to avoid a loss of signal level when the amplifier is fed with a high resistance source. The main function of the last, or output stage of the cascade amplifier is to provide a low output resistance in order to avoid the loss of gain where a low-valued load resistance is connected to the output stage. The cascode configuration typically is a two-transistor amplifier stage configuration that combines a common-emitter transistor circuit and a common-base transistor circuit (the common-source and the common-gate circuits in the field effect transistor (FET) case). Switching gain stages of a cascade or cascode amplifier however, will affect the amplifier's overall input and output impedance, as well as the noise figure.
Alternatively, the gain of the amplifier stage may be varied by changing the gain of one or more of the individual stages. As previously stated, varying the gain of one or more of the individual stages results in variations in the input impedance, output impedance, noise figure, and the generation of third-order harmonic signal noise. Consequently, the current methods can be unsatisfactory for maintaining required levels of input and output impedances for the amplifier gain stage and for maintaining a required noise figure level while varying the gain of the variable gain amplifier stage.